1. Field of the Invention
The present invention relates to an image reading apparatus for two-dimensionally reading an image with line sensors, and more particularly to an image reading apparatus whose resolution has been increased without increasing the number of photoelectric transducers of line sensors.
2. Description of the Related Art
There have widely been used image reading apparatuses including photomultipliers or charge-coupled devices (CCDs) for reading an image on a document to generate an image signal. Since the image reading apparatuses divide an image into a finite number of pixels and read image information from the pixels, there is a certain limitation on the spatial frequencies of images that can be reproduced from image information read by the image reading apparatus.
If an image is read using a photomultiplier, then it is possible to increase the resolution of the read image by increasing the frequency of a synchronizing signal used in the reading process.
If an image is read using a CCD, however, then since the number of pixels of a produced image signal is limited by the number of the photoelectric transducers included in the CCD and also since the area of the photosensitive section of the CCD is limited depending on the areas of the photoelectric transducers, the resolution of the CCD is lower than the photomultiplier. Specifically, the spatial frequency of an image that can be reproduced by a CCD is 1/2P (Nyquist frequency) where P is the distance between the centers of photoelectric transducers, and any spatial frequencies higher than 1/2P give rise to noise.
When images having a spatial frequency in the vicinity of 1/2P are read by a CCD, the CCD responds largely differently depending on the phase of the images with respect to the photoelectric transducers even if the images are the same as each other. For example, as shown in FIG. 12 of the accompanying drawings, when identical sinusoidal images A, B which are 90.degree. out of phase with each other with respect to photoelectric transducers P1, P2, . . . of a CCD are read by the CCD, an image signal generated by the CCD is not representative of the images A, B because the images applied to each of the photoelectric transducers P1, P2, . . . are averaged. FIG. 13 of the accompanying drawings shows the relationship between the spatial frequency and the CCD response at the time the CCD reads identical sinusoidal images that are 20.degree. out of phase with each other. FIG. 13 indicates that the CCD response with respect to the phase differs largely as the spatial frequency is higher. An image reading apparatus known as a scanner for producing film plates for use in printing outputs images at high magnification ratios. In such an image reading apparatus, images having higher spatial frequencies are largely affected by response differences due to phase differences, and such response differences appear as moire patterns, for example, in the outputted images.
Japanese laid-open patent publication No. 3-236687 discloses an attempt to solve such a problem. According to the disclosed arrangement, a solid-state imaging device composed of an area sensor is displaced in four directions, i.e., upward, downward, leftward, and rightward, and pixel signals generated by the solid-state imaging device in these respective four positions are arranged in a given sequence for thereby increasing the apparent number of pixels of the solid-state imaging device, thereby increasing the resolution.
The solid-state imaging device comprises an area sensor, and pixel signals generated by the solid-state imaging device in respective four positions are stored in respective four image buffers and subsequently need to be combined in an image synthesis memory. Therefore, a considerably large memory capacity is required for these memories. The area sensor suffers a structural limitation which makes it difficult to provide a higher resolution than line sensors. It has been impossible to remove effects which the phase has on the image at higher spatial frequencies in the area sensor.